The invention relates to paper-making machines. Specifically, the instant invention includes sensors and an appertinent system for detecting and controlling the weight of paper manufactured by a paper-making machine, and a method for accomplishing the same.
Paper-making machines and quality control instrumentation therefor are well known in the art. Known quality control instrumentation generally includes some form of scanning sensor which is moved across the paper, in what is known as a cross-direction scan, to provide a value which is indicative of the bone-dry weight of the paper stock being manufactured in the paper-making machine. As the instrumentation is scanning the paper in a cross-direction, the paper is moving through the machine (machine-direction) and the net result is that the quality control instrumentation detects the weight of the paper in a zig-zag course down the length of the paper web formed in the machine.
Even where multiple scanning sensors are provided, the scanning system generally requires about ten or more cross-direction scans in order to get an accurate representation of the variation of cross-direction weight of the paper web. Additionally, the scanning sensors are typically located close to the take-up reel of the paper machine which results in a considerable time delay between the paper-forming suspension leaving the headbox, transitting the machine and arriving at the sensor. Even after the scanning sensor has been calibrated and errors corrected, there will still be a delay between the detection of an unacceptable variation in paper thickness at the sensor and the input on an appropriate control signal to the slice-lip actuators at the headbox. This delay may result in the production of an unacceptable quantity of paper which is either too light or too heavy.
An object of the invention is to provide an apparatus and method for determining short term cross-direction and machine-direction variations in a web of paper and correcting such variations.
Another object of the invention is to provide an optical measuring system which extends continuously across the width of a paper web and which will detect cross-direction and machine-direction variations in a paper web.
A further object of the invention is to provide an optical scanner which is calibrated by a scanning sensor.
Another object of the invention is to provide a system and method for making a direct measurement of fiber and water content of a paper forming web.
Yet another object of the invention is to provide relatively instantaneous control inputs to slice-lip actuators on a paper-making machine headbox.
Another object of the invention is to provide a stationary sensor which is capable of defining short term cross-direction and machine-direction variations in the paper web.
Still another object of the invention is to provide a stationary sensor which is operable to adjust a stock valve of a paper-making machine for machine direction control.
A further object of the invention is to provide means for determining and inferring a bone-dry weight of a paper weight as a function of web opacity.
The system of the invention is intended for use on a paper-making machine having a headbox therein with a slice-lip mounted thereon for forming a moving web of material. Plural slice-lip actuators for setting the slice slip gap to control the amount of material in the web are mounted on the headbox. Plural stationary optical sensors are located at a first station and extend across the width of the web in a continuous array for detecting the transmissivity of the web in a one-to-one relationship with the slice-lip actuators at an aligned location downstream of each actuator. Each sensor includes plural light detectors therein which are operable to generate a first transmissivity signal representing the transmissivity of the web for discrete regions thereof adjacent each light detector corresponding to a given actuator. A calibration device is provided for sensing a selected web parameter for a discrete region of the web and is operable to generate a calibration signal indicative of the selected parameter. The system includes a computer which is operable to compare each transmissivity signal and a corresponding portion of the calibration signal for a discrete region of the web and to generate a control signal which is a function of the transmissivity signal as calibrated by the calibration signal. The computer is further operable to generate related actuator signals from the control signals for adjusting each slice-lip actuator.
These and other objects and advantages of the invention will be more fully appreciated as the description which follows is read in conjunction with the drawings.